1. Field of the Invention
The present invention relates to a semiconductor device, in particular, suitable for an oscillation control circuit in semiconductor devices for microcomputers.
2. Description of the Related Art
A microcomputer (hereinafter, sometimes refer to as a “micon”) starts operations of a CPU and the like existing in the inside, after a period of waiting time for stabilization of its oscillation after, for instance, turning on the power. The waiting time for stabilization of oscillation for a micon is provided not to operate (start) the CPU and the like until the oscillation of an inputted clock signal is stabilized even though the oscillation circuit starts the oscillation movement, that is, during an unstable period of a clock signal immediately after starting of the oscillation movement.
The waiting time for stabilization of oscillation for a micon is required to satisfy requirement of an oscillation stabilization period of time (a period of time until an oscillation state is stabilized) of a resonator connecting outside, and the oscillation stabilization period of time is determined for every resonator according to the product specification. Accordingly, in the waiting time for stabilization of oscillation for a micon, a period of time having a sufficient margin appropriate for general purposes is established in advance to be able to correspond to various resonators different in characteristics. Accordingly, a conventional micon cannot start operation of a CPU or the like during the same waiting time for stabilization of oscillation as the waiting time when a resonator having a long stabilization time for oscillation, even though a resonator having a short stabilization time for oscillation is used.
In recent years, in a micon having a communication function connected with a network, there is a demand that the micon be kept waiting in a low power consumption state such as a stop mode or a sleep mode, and when receiving data, it returns to a normal operating state from the previous state to make the received data available, etc. However, if the waiting time for stabilization of oscillation of a micon is too long, the data cannot be received correctly, which may cause the so-called frame loss. Accordingly, reduction of the time required for stabilization of oscillation is strongly demanded.
As a technology to improve a waiting time for stabilization of oscillation according to this demand, a technology to reduce a time required for stabilization of oscillation by being provided with a self-exciting oscillation circuit and an external oscillation circuit connected with an external resonator and by supplying an output of the self-exciting oscillation circuit to the external resonator to accelerate the oscillation at the time of starting the oscillation has been disclosed (see, for instance, Japanese Patent Application Laid-open No. Hei 9-93040). In a micon provided with two oscillation circuits different in oscillation stabilizing time from each other, another example of the technology to activate respective resources beginning from a resource that has elapsed its oscillation stabilization time in sequence to an operation state by counting the output signals of the oscillation circuits with each other to judge whether or not the oscillation stabilization time has passed, is disclosed (see, for instance, Japanese Patent Application Laid-open No. Hei 6-138975).